1. Field of the Invention
The present invention generally relates to electronic devices and more particularly to a method for packaging devices using thin bonding regions.
2. Description of Related Art
Electronic devices fabricated in a large scale integrated (LSI) manner by thin film deposition, etching, etc., need to be packaged so as to be easily handled, and so as to be reliable in the variety of environmental conditions that are typically present in electronic systems. In the case of most simple electronic devices, polymer materials may be applied directly on the surface of a finished device for packaging. In the case of devices that are based on acoustic waves or mechanical motion, the part of the device that contains these acoustic waves must be isolated from intimate contact with packaging material, or the acoustic wave may be corrupted. The isolation can be achieved by forming a desired sized cavity of desired depth over the devices. It is preferred to form these cavities at the wafer scale, such that a multitude of devices are made in a batch process, lowering the cost per part. One way to do this is to prepare a capping wafer to be bonded to a wafer that is prepared with devices. These wafers can then be bonded together, and thereafter the devices can be singulated (e.g., separated into individual devices).
One such type of device is a thin film resonator (TFR). In these devices, electrical signals are transformed into acoustic waves in a piezoelectric material, the acoustic waves resonate in a prepared structure where certain frequencies are reinforced, and electrical signals are again produced by the now changed acoustic waves to produce a filtering function. For example, such a TFR is disclosed in U.S. Pat. No. 6,323,744 to Barber et al. entitled “GROUNDING OF TFR LADDER FILTERS”, the entire contents of which are hereby incorporated herein by reference.
A conventional wafer stack is described in FIGS. 1-2 of U.S. Pat. No. 6,106,735 to Kurle et al, where a cap wafer 3 is fabricated with webs 4 and contact holes 9. These webs 4 are sealingly bonded to a substrate surface containing sensor components, such that gaps (caverns) are formed between adjacent webs.
Each of these webs have bonding strips 5 (e.g., frit glass seal strips) deposited on webs 4. These strips 5 are used for bonding the cap wafer 3 to a substrate wafer 1. In particular, these frit glass seal strips 5 have a line width 21 of about 500 micrometers, which corresponds to the header width of a particular web 4.
The frit strips shown in the prior art are typically applied with a screen printed process, and a resulting minimal linewidth is typically quite large, greater than 125 microns, as indicated in Kurle et al. This is undesirable, since these substrate surfaces (to which the cap wafer is to be attached to) contain many electrical components or sensors such as thin film resonator (TFR) components and/or filter components. From these components or devices, interconnects must be run under the frit seal in the cap wafer, and in the above example in Kurle et al., an additional 500 extra microns of “runner” are needed in order to get underneath the frit. This introduces resistive losses and additional inductive and capacitive parasitics that limit device performance. Additionally, the excessive frit width of the prior art also introduces a significant increase in die size; thus there are fewer sensory devices produced per wafer.